A solid catalyst component that includes magnesium, titanium, internal electron donor, and a halogen as essential components has been used when polymerizing an olefin (e.g., propylene). A number of methods that polymerize an olefin or copolymerize olefins in the presence of an olefin polymerization catalyst that includes the solid catalyst component, an organoaluminum compound, and an organosilicon compound, have been proposed.
Polypropylene compositions are mainly controlled by the kind of solid catalyst and polymerization conditions. From a point view of polypropylene stiffness (i.e. flexural modulus, for example), it is strongly correlated to the isotacticity of polypropylene; generally, highly isotactic polypropylene shows high flexural modulus. This type of polypropylene is utilized for automotive, for example.
For example, Patent Document 1 and 2 disclose olefin polymerization catalysts with high isospecificity. Polypropylene produced with these catalysts above shows high flexural modulus. However, there is still scope for improvement in flexural modulus of polypropylene.
On the other hand, some substituted malonate compounds are found to be an excellent internal donor to give polypropylene with high isotacticity. For example, Patent Document 3 discloses olefin polymerization catalyst including alkylidene malonate compound as an internal electron donor. And Patent 4 discloses olefin polymerization catalyst including dialkyl-substituted malonate compound as an internal electron donor. With these catalysts, polypropylene containing low xylene-soluble fraction (atactic polypropylene) can be obtained. However, these types of polypropylene cannot be applied to automotive use due to insufficient isotacticity.
Furthermore, Patent Document 5 discloses olefin polymerization catalysts containing succinate compound as an internal electron donor. When using the catalyst system above, the resulting polypropylene has a broad molecular weight distribution, and inferior isotacticity as compared with a polypropylene produced using a solid catalyst that utilizes a phthalate compound as an internal electron donor. Therefore, it is difficult to obtain a polypropylene that exhibits high flexural modulus.